The fidelity of protein biosynthesis depends on the accurate transcription and translation of the genetic code carried by chromosomal DNA, and on the specific attachment of the proper amino acid to its cognate transfer RNA. This research will focus on the chemical mechanisms by which the aminoacyl-tRNA synthetase enzymes recognize their substrates and catalyze the aminoacylation reaction. Studies of binding and recognition sites on the enzymes themselves will be designed to complement previous work, which has dealt mainly with the identification of specific sites on the tRNA molecules which interact with their respective synthetases. The tryptophanyl-tRNA synthetase of E. coli has been selected as the initial candidate for detailed experiments because: (a) it is available in large quantities from a bacterial strain which produces 55-fold elevated amounts of this enzyme; (b) a part of the amino acid sequence of the E. coli Trp-tRNA synthetase has been determined; and (c) this enzyme is a relatively small member of the synthetase class, having two identical subunits of 37,000 molecular weight, and is therefore amenable to detailed chemical investigation. Initial studies will identify which of the histidine-containing peptides interact directly with cognate or noncognate tRNA's. In addition, affinity labeling and NMR spectroscopy will be used to characterize the binding of substrates at the active site. Later work will deal with structural features in three dimensions which govern both specific recognition, and the mechanism of catalysis.